The present invention relates to a noise eliminating circuit, and more particularly, to a noise eliminating circuit for eliminating noise (i.e., thermal asperity) in an amplification circuit that is produced when a magneto resistive (MR) head contacts a hard disk, which amplifies reproduction signals of the hard disk.
In a conventional hard disk device, heat is produced when an MR head contacts a hard disk while reading data. This increases the resistance of the MR head and results in a reproduction signal ST that includes low frequency noise, which is referred to as thermal asperity (TA), as shown in FIG. 1.
FIG. 2 is a schematic block diagram showing a first prior art thermal asperity compensating circuit 81, which eliminates thermal asperity. The compensating circuit 81, which is connected between a first read amplifier 82 and a second read amplifier 83, includes a capacitor C, a resistor R, and a switch SW. The compensating circuit 81 functions as a bypass filter. When thermal asperity is detected, the switch SW is activated and the compensating circuit 81 eliminates the thermal asperity.
FIG. 3 is a schematic block diagram showing a second prior art thermal asperity compensating circuit 84. The compensating circuit 84 is connected between a first read amplifier 82 and a second read amplifier 83 and includes a delay circuit 85, an envelope waveform generating circuit 86, a low-pass filter 87, and an operational amplifier circuit 88.
An amplified reproduction signal generated by the first read amplifier 82 is delayed by the delay circuit 85 and then provided to the operational amplifier circuit 88. The amplified reproduction signal is also provided to the operational amplifier circuit 88 via the envelope waveform generating circuit 86 and the low-pass filter 87.
When thermal asperity is included in the amplified reproduction signal output by the first read amplifier 82, the thermal asperity component is provided to the operational amplifier circuit 88 via the envelope waveform generating circuit 86 and the low-pass filter 87. The operational amplifier circuit 88 detects this noise and then removes the thermal asperity component from the amplified reproduction signal provided by the delay circuit 85 and generates an amplified reproduction signal from which the thermal asperity is eliminated.
The first read amplifiers 82 located upstream of the thermal asperity compensating circuits 81, 84 also amplify the thermal asperity component. Accordingly, the first read amplifier 82 may be saturated by the thermal asperity component. A circuit for preventing saturation may be provided in the first read amplifier 82. This would, however, complicate the circuit configuration of the first read amplifier 82. Further, after eliminating thermal asperity, a relatively long time would be necessary to terminate the saturated state. In other words, a certain length of time would be necessary for the first read amplifier 82 to start functioning normally again.